HMS researchers have shown that the abundant microbial life in our gastrointestinal tracts, so essential to our maintaining a healthy immune system, co-evolved with their human hosts to such an extent that they provide no immunological benefit to any other mammal. The study sheds light on the hygiene hypothesis, which states that environments kept exceedingly clean with antibacterial products might thwart the immune protection offered by microbes. The study appeared in the June 22 issue of Cell.